Dentine forms the frame of a tooth. It surrounds the pulp and is covered by enamel on the crown and cementum on the root. The cementum does not always meet the edge of the enamel at the cementum-enamel junction. In such cases, dentine, which is not protected by another type of hard tissue, is exposed at the neck of a tooth. In the young, this area is usually covered by gingiva which covers this particular zone of dentine and prevents its exposure to irritation from the oral cavity.
Dentine consists of an extracellular matrix, which is formed by odontoblasts lining the pulp cavity. When dentine is formed and it becomes thicker, each odontoblast leaves behind a cell extension i.e. an odontoblast process. These processes remain inside the developing tissue and form dentinal tubules, which extend from the enamel-dentine/cementum-dentine border into the pulp. When exposed, open dentinal tubules form a link between the dentine surface and the pulp.
The structure of dentine is shown in detail in FIG. 1.
Once the tooth has stopped growing, odontoblasts continue their function and form secondary dentine on the pulp side of the tooth. They also form hard tissue i.e. intratubular dentine which gradually closes the tubules around the receding odontoblast processes (FIG. 2.). The level of mineralisation of intratubular dentine is significantly higher than that of intertubular dentine. The mineralisation of dentinal tubules is a very slow but natural process, associated with the ageing of a tooth. The slow pace and unpredictability of the process are manifested as problems in various clinical situations.
The hydrodynamic pain transmission mechanism of exposed dentine can be described as follows:
Dentinal tubules are 1-2 .mu.m in diameter. When a section is cut perpendicular to the dentine surface, there are around 30 000-40 000 dentinal tubules per mm.sup.2. A dentinal tubule is filled with an odontoblast process, surrounded by fluid from the pulp. A very strong capillary force prevails in open and exposed dentinal tubules. Consequently, fluid mechanically removed from the opening of the tubule is quickly replaced by fluid flowing out from the pulp. Similarly, substances with strong osmotic pressure (e.g. sweet solutions) cause an outward flow of fluids in the tubule, which in turn, leads to transformations of the odontoblasts lining the pulp chamber and tissues surrounding ondontoblasts, thus activating the nerve endings. On the other hand, irritation transmitted to the pulp may be caused by the inward flow of the fluid in the dentinal tubule. In practice, the hydrodynamic mechanism refers to the fluid flow in a dentinal tubule caused by a stimulus and the resulting hydraulic vibration in the pulp. According to current opinion and experience, the closing of the tubule and the resulting complete or partial prevention of fluid flow would lead to a reduction or even elimination of nerve activation, sensed as pain regardless of the primary stimulus (1). The pain transmission mechanism is illustrated in FIG. 3.
Clinical Problems
The exposure of dentine and dentinal tubules leading to the pulp may be the result of caries. The hard tissue (enamel/cementum) protecting the dentine is destroyed during the decay process. The situation leads to the known painful symptoms associated with cavity formation. The pain results from irritation which is transmitted to the pulp nerve endings through the dentinal tubules. During the decay process, partial mineralisation of tubule contents is usually observed. This is a consequence of the high calcium and phosphate ion concentration caused by demineralisation processes of the enamel and dentine in the close vicinity of intact dentine tissue. Strengthening of this phenomenon and its incorporation into the properties of filling materials would be desirable.
Exposure of dentine is also often associated with the periodontal diseases of a tooth, and also with the normal ageing process and dental hygiene habits. In certain cases, even in the young, the gingiva recesses or teeth erupt so that the necks of the teeth become exposed. This itself may lead to strong painful symptoms, in patients whose dentine was not covered by the protective and non-sensitive cementum in the first place. More often, however, exposure of dentine is associated with wrong toothbrushing habits, either using too heavy a hand, a coarse brush or the wrong technique. This results in worn and hypersensitive teeth, which affects the person's tolerance of hot/cold/bitter/sweet drinks and food, makes breathing more difficult in the hot and cold, and also interferes with proper oral hygiene.
Infections in the periodontal tissues and, especially, the treatment of periodontal infections, generally lead to gingival recession and exposure of dentine, often of fairly large areas. As successful therapy involves the removal of the root cementum protecting dentine, and polishing, which wears the teeth further, it is obvious that the general consequence of such treatment (25%) is the exposure of tooth necks and hypersensitivity. If the patient is fairly elderly and their dentinal tubules exhibit a substantial layer of highly mineralised intra-tubular dentine, the post-treatment pain is less severe. On the other hand, the pain resulting from the treatment may, in certain cases, be strong, it can continue for weeks and may require the use of analgetics. In the most extreme cases, irritation may lead to infection and, subsequently, to the death of a tooth and root canal treatment. In association with dental care, hypersensitive teeth is an irritating problem which occurs frequently, and should have a simple and inexpensive cure (2). FIG. 4 shows a tooth and the area with the exposed dentinal surface.
Known Methods of Treatment
Tooth ache, resulting from cavity formation refers, regardless of its similarities to hypersensitive dentine, to a different problem area. Sensitivity associated with caries, and pain caused by irritation is usually treated by fillings. At the bottom of the prepared cavity, a commercially available preparation is placed against the pulp, the biologically active component of such preparation is usually calcium hydroxide (CaOH.sub.2). At the cell level, the strongly alkaline calciumhydroxide first induces irritation, which leads to the necrotisation of the tissue. Over a longer time span, however, it promotes the healing process. The result of the treatment is the formation of reparative secondary dentine. The formed tissue layer separates the pulp from the damaged area or the filling, but its effect on the mineralisation of dentine tubules is minimal.
During filling, the dentinal tubules can also be closed by glass ionomer cement, or with different preparations based on polymer chemistry (binder plastics, resins, dentine adhesives). These substances close dentinal tubules mechanically and improve the retention of the filling being prepared. Fluoride which is released from the glass ionomer cement may, in theory, have a positive effect on the mineralisation process of dentinal tubules. However, there are no research results of the possible clinical role of this phenomenon. Plastic-based preparations have no biological effects which promote the healing process and/or the formation of hard tissue between the pulp and the damaged and restored area.
The epidemiological data describing the extent of the problem caused by hypersensitive dentine and the need for its treatment is limited. In practice, dentine hypersensitivity is a common and typically highly variable problem. Because the symptoms are linked closely to the behaviour of a person and, in the long run, disappear; and because there are commercially available toothpastes, which may lower the sensitivity, the actual extent of the problem is difficult to define solely on the basis of how often people seek for professional help at their dentist's for this particular problem. In association with the treatment of inflamed gingivae and supportive tissues of the teeth, the need to offer relief for hypersensitive teeth is often acute.
Nowadays, two different concepts of treatment for hypersensitive teeth are available. These treatments are based on either raising the pain threshold of a tooth, or on the formation of a protective mineralisation precipitate either on the surface of a tooth or, preferably, in the dentinal tubules. In addition, the treatment involves gentle (possibly chemical) plaque control, diet guidance and confirmation that the irritation threshold of the pulp is not lowered by masticational overload or a poor filling which maintains chronic infection in the pulp.
For a long time, some toothpastes have contained substances designed to give relief to hypersensitive teeth (3, 4, 5, 6). The goal has been either the denaturation (formaldehyde) of the contents of the dentinal tubule or the formation of mineral precipitates (strontium chloride, fluorides, abrasives).
Sodiumfluorophosphate may infact have some therapeutic effects. Potassiumnitrate and potassiumcitate reduce the irritability of the pulpal nerves without affecting the actual contents of the dentinal tubules (7). The problem with substances which merely raise the activation level of nerve endings (including corticosteroids) is that they do not strengthen the tooth and the pulp remains exposed to hydrodynamic irritation even after treatment. Consequently, the therapeutic effect of such treatment only lasts for a short time. Research results concentrating on the therapeutic effects of toothpastes are highly contradictory. On one hand, placebo effects, and on the other hand, pain relieving effects up to 80% have been reported. Generally, the problem with toothpastes is that they act very slowly, often only after several weeks of use. Thus, toothpastes may be suitable for home treatment of subacute problems. It is, however, necessary to find more powerful and quicker-acting methods for acute pain.
In clinical situations, today, the most common method for treating hypersensitive teeth is to use fluorides--either sodiumfluoride or tin fluoride in 2-10% mixtures (8, 9, 10). Fluoride can also be applied topically to the surface of a tooth in combination with varnish-based substances (Duraphat (11)). The varnish prolongs the effect of fluoride, and at the same time, the varnish itself may have some tubule blocking effect. At least over a short time-span, fluoride preparations have been shown to have positive therapeutic effects. Recently, some attention has been paid to the acidity of these preparations and irritation related to this acidity. It is thought, however, that problems caused by the acidity in the pulp can be relieved by alternating the use of calciumhydroxide and fluoride. This treatment has been empirical. In principle, results have been positive. Objective research data of the effects of the treatment or its permanence is not available. The presence of alkaline calciumhydroxide may, however, complicate the formation of the practically insoluble, and therefore, desired tinfluorophosphate; and instead, favour the formation of calciumfluoride which dissolves in neutral environment. In such conditions, the treatment may be effective, but temporary.
Solutions which assist the remineralisation of the tooth surface may also reduce sensitivity when used for long enough. Due to their watery nature and slow effect, this method--or those with the toothpastes--is not very good for the treatment of acute pain. For example, two mineralisation solutions--A and B--have been used. Solution A contains 6 MM PO.sub.4 and B mM Ca. In addition, both solutions contain 0.15 mM NaCl and 5 ppm F. 10 ml of solution A and 10 ml of solution B are mixed in a glass immediately before use. The mixture of mineralisation solution, diluted in water is rinsed in the mouth for 1-2 minutes, and spat out. It is recommended that this procedure is repeated twice a day preferably after tooth brushing.
Potassiumoxalate (K.sub.2 C.sub.2 O.sub.4 or KHC.sub.2 O.sub.4, 3-30%) has also been used in the treatment of hypersensitive teeth. The idea behind the use of these chemicals as a therapeutical agent, is based on the ability of oxalate to precipitate calcium residing on the surface of a tooth or in the dentinal fluid. In this reaction, crystals which obstruct the transmission of hydraulic stimulus from the tooth surface into the pulp, are formed. A major part of the precipitate thus obtained usually dissolves within a week, but the diameter of dentinal tubules remain smaller than before treatment. Long-term effects of the treatment are still to be confirmed (3, 12, 13). More permanent results have been obtained with ferro-oxalate (6%) than with other oxalate treatments (14). At least one research report, however, has found acidic table salt solutions more effective than oxalate against hypersensitive teeth.
Products based on plastic polymers (resins, dentine adhesives) and cyanoacrylate effectively block dentinal tubules (15). At least over a short time-span, they remove pain and protect the pulp from the immediate irritation. These substances, however, cannot be regarded as biological, since they do not lead to a natural healing process and to the mineralisation of dentinal tubules. Dentine adhesives have also been found to be highly allergenic. The dental care personnel, however, are the major target of their allergenic effect. Additionally, acrylate, meta-acrylate and cyanoacrylate compounds have been found to be irritants, and also genotoxic and carcinogenic both in animal experiments and cell culture studies. Plastic-based "coating" also forms a microbe retentive surface at the area of gingiva-tooth junction which easily leads to the recurrence of the recently treated disease. Thus, the use of dentine adhesives, especially in periodontal patients, cannot be regarded as meaningful. When considering filling materials, glass ionomer cement has also been proposed for the treatment of hypersensitive teeth (11). Advantages of glass ionomer cement are its properties to bind dentine and to release fluoride. In practice, glass ionomer cement is difficult to use, especially in the treatment of widely exposed dentine. However, it is suitable for the treatment of distinctly defined and relatively deep abrasion lesions which are found clearly above the gingiva margin.
Older literature suggests that dentinal tubules should be blocked by using silver nitrate. The results of using this substance have been highly variable. In addition, silver nitrate is a highly staining agent. The treatment of root surfaces first with zinc chloride and then with potassium ferrocyanide have also been proposed. The treatment yields a protective precipitate on the surface of a tooth. The results have been reported to be satisfactory. If swallowed by mistake, however, the agent is toxic.
The use of a NdYAG laser has been proposed as a potential new approach for closing the dentinal tubules. Preliminary results with this treatment are promising. The mechanism behind the treatment, the permanence of the treatment and its possible adverse effects on the pulp are still to be confirmed.
The heterogeneity of possible treatments presented above, gives a good picture of a dentist's real and realistic choices in the effective treatment of hypersensitive teeth. Even after several years of experiments, none of the offered alternatives has proved to be any better than the other, and thus a predominant method of treatment. Common to all the offered methods, is that they do not consciously aim at producing an apatite-based compound (Ca.sub.10 (PO.sub.4).sub.6 X.sub.2, in which X is either a hydroxyl or fluoride)--chosen by nature--to close the dentinal tubules. Generally, the goal has been to make any type of precipitate to block the dentinal tubules as quickly as possible (3, 16, 17). None of the tested methods of treatment has aimed at contributing to the crystallisation process itself, by simultaneously adding calcium and/or phosphate which essentially participates in the formation of precipitate/crystals. Therefore, at the background of the partly contradictory results reported may be the availability of these essential ions during the short period of treatment. On the other hand, some of these treatments have sought results through the cumulative effects of short-term application (e.g. toothpastes and remineralisation solutions). An exception to these treatment concepts is the use of fluoride containing preparations, although their effect is not based on increasing the availability of calcium or phosphate on the area to be treated.
The purpose of the present invention is to eliminate the above mentioned problems and present a new, effective preparation for reducing or inhibiting pulpal irritation i.e. hypersensitivity of a tooth. The preparation also provides a new method for effectively strengthening a tooth.